1. Field of the Invention
The invention relates to the field of methods and apparatus for providing illumination and in particular to methods and apparatus that are energy efficient, typically in excess of 90% energy efficiency and thus are specially suitable for low power light sources, thereby allowing replacement of higher power, less energy efficient light sources with no loss in illumination intensity.
2. Description of the Prior Art
The typical LED emitter radiates its light energy in a nearly hemispherical beam pattern. Most LED applications require substantial modification of this output beam profile to provide useful energy. The typical means of modifying this LED energy in the body of prior art are conic reflectors, lenses, or a combination of these.
One of the most widely used means of developing a beam from an LED source is the use of a hemispherical ball lens. The lens is typically placed with its center approximately one radius distance off the center of the emitter. To provide a means to place this lens above the emitter, a cylindrical surface is incorporated that extends from the base of the hemisphere backward toward the emitter. This solution utilizes about 60% of the energy available from the LED and creates a relatively smooth spot whose beam angle is based on the ratio of LED emitter size to ball lens diameter. This is the method utilized in the standard 3 mm and 5 mm LED packages. This is a relatively efficient optical solution for a small device, but not optimum in collection efficiency. A major drawback of this method is chromatic aberration generated by the design that is particularly undesirable when utilized in ‘white’ LED systems.
Another common technique is the use of a single surface of rotation-conic reflector. When the cross-section is a parabola or an ellipse with a long focus the beam produced is much brighter in the center than in its periphery. While very useful for flashlights and some other devices, this is not appropriate for all LED illumination needs. When the cross-section is an ellipse with a short focus a portion of the resultant beam crosses its centerline axis and creates a center zone or ‘hole’ of less energy in the illumination pattern at a distance. Attempts to generate a smooth beam from either of these configurations, generally with the addition of a diffuser, result in a substantial loss of efficiency.
A modest improvement to the ball lens approach is an integrated optical system incorporating features of both the ball lens and a conic reflector in a single optical device. The energy pattern radiating from this approach suffers similar chromatic aberration, or color separation, with ‘white’ LEDs as does the ball lens system. This method is slightly more efficient than the ball lens device.
A common method of creating a beam from a light source in the prior art is the CPC, Compound Parabolic Concentrator. This means was actually developed to collect energy toward a focus for solar line collectors, but more recently has been added to the art of reflector design.